1. Field of the Invention
The present invention relates to nuclear power plants, such as, without limitation, pressurized water reactor (PWR) type nuclear power plants, and in particular to a tool for unlatching and removing a control rod drive shaft in a nuclear reactor vessel.
2. Description of the Related Art
In nuclear power generation, a reactor vessel is the primary vessel wherein heat is generated for producing steam. The reactor vessel typically includes a flanged body having a flanged, removable upper closure head bolted atop its upper portion for forming a sealed enclosure. Fuel pellets, which are located within fuel assemblies, are positioned within the reactor vessel for producing a controlled nuclear fission reaction which, in turn, generates heat. The heat generated by the fission reaction heats borated water that is contained within the reactor vessel. Process piping, generally referred to in the art as a primary loop, is attached to the reactor vessel. The heated borated water flows out of the reactor vessel and passes through the primary loop to a steam generator for transferring its heat to a secondary loop, wherein steam is produced for ultimately producing electrical power, as is well known in the art. The borated water then returns to the reactor vessel via the primary loop where the above described process is repeated. In a pressurized water reactor (PWR), and in contrast to a boiling water reactor (BWR), pressure in the primary loop prevents the borated water from boiling within the reactor.
The rate of the fission reaction taking place within each fuel assembly is regulated by means of an associated control rod assembly. Each control rod assembly is formed from an array of stainless steel tubes containing a neutron absorbing substance, such as silver, indium or cadmium. These stainless steel tubes (known as “rodlets” in the art) are suspended from a spider-like bracket, and a control rod drive shaft (CRDS) is connected to the spider-like bracket. Each CRDS is also coupled to a control rod drive mechanism (CRDM) carried by the closure head. Each CRDM is structured to either insert or withdraw the rodlets of the associated control rod assembly deeper into or farther out of the associated fuel assembly in order to modulate the amount of heat generated thereby.
Periodically, nuclear reactors must be refueled, a process wherein a fraction of the fuel assemblies of the reactor are replaced. During the refueling of a nuclear reactor, the closure head is removed, the reactor vessel is flooded with water and the upper internals of the reactor vessel are removed. When this is done, however, the rodlets need to remain in place within the reactor vessel. Thus, prior to removing the upper internals of the reactor vessel, each CRDS (which is carried by the upper internals) must be disconnected from the cluster of rodlets to which it is attached so that the rodlets will not be carried away with the drive shaft but instead will remain in place. More specifically, each spider bracket has a grooved circular ferrule hub and the bottom of each CRDS has a pair of fingers that are received in the ferrule to couple the CRDS to the spider bracket. This connection must be released so that the CRDS can be removed from the reactor vessel while leaving the rodlets in place.
For some time, a prior art CRDS unlatching tool has been used to release the connection between a CRDS and a spider bracket. That tool uses a first pneumatic mechanism to actuate a first set of latch fingers provided on a button shaft which engage a top portion of the CRDS and disengage the CRDS from the spider bracket (it causes the fingers to be released from the ferrule hub) and a second pneumatic mechanism to actuate a second set of latch fingers which engage an outer surface of the CRDS and allow it to be held by the tool while it is removed. The problem with this prior art tool is that it undesirably permits a tool operator to inadvertently actuate the air cylinders of the second pneumatic mechanism while the tool is latched onto the CRDS, thereby allowing the CRDS to fall out of the tool. As will be appreciated, this has the potential to cause costly damage to the CRDS itself and to nearby equipment and/or injury to nearby personnel.